1. Field of the Invention
The present invention relates to a photo mask used in photo lithography and a repair method of the same.
2. Description of the Related Prior Art
In a projection exposure apparatus for irradiating a photo mask on which an original pattern is drawn by an illuminating system, and projecting the pattern on the mask onto a wafer, the pattern to be projected is demanded to be higher in definition. As the method for realizing higher definition, recently, a method of using a phase shift mask for applying a phase difference to the exposure light is being noticed.
The phase shift mask is explained by reference to FIG. 11.
To provide exposure light with a phase difference, the photo mask is composed of a light-shield pattern 2 and a phase shifter 3 formed on a transparent substrate 1. The phase shifter 3 is a pattern using transparent film for applying a phase difference of 180 degrees to the transmission light into the phase shift mask. Using the phase shift mask of such structure, a finer resist pattern on the wafer can be formed. The phase shift mask is complicated in structure as compared with the conventional photo mask composed only of light-shield film pattern, and strict control is required in the thickness of the transparent film, refractive index, etc.
In particular, if there is a defect in the transparent film pattern, it is difficult to repair it.
Hitherto, as the defect repair method of such transparent film pattern, a method presented in the collected papers for the 51st General Meeting of Japan Society of Applied Physics, 27p-ZG-10,p. 493 (1990) has been proposed. This is a method using a transparent film for mask repair preliminarily on a mask substrate. Another method is proposed in the collected papers for the 52nd General Meeting of Japan Society of Applied Physics, 12p-ZF-5, p. 605 (1991). This is a method of etching the defect of the transparent film using a focused ion beam (FIB).
The former method of repair using the transparent film is explained by referring to FIG. 12. A light-shield film pattern 2 is formed on a transparent substrate 1. Further thereon is formed a transparent film for applying a phase difference of 180 degrees to the transmission light, which is called a sub-shifter 4. A transparent film pattern is formed thereon as a phase shifter 3. A dent defect 5 is formed in the phase shifter 3 (FIG. 12 (a)). A resist 6 is applied on the entire surface of the transparent substrate 1, and the window of the resist 6 is formed in a region including the periphery of the defect 5 by exposure and development (FIG. 12 (b)). Later, using this resist 6 as a mask, the phase shifter 3 and sub shifter 4 are removed by dry etching (FIG. 12 (c)). In this process, the phase of the light passing through the etched region is same as in ordinary exposure. On the other hand, the light passing through the phase shifter 3 is provided with a phase difference there, and when passing through the sub-shifter 4, a further phase difference of 180 degrees is given. That is, the light passing through the phase shifter 3 is delayed in phase by 180 degrees, and is further delayed in phase by 180 degrees in the sub-shifter 4. In total, the phase is delayed by 360 degrees. It means it is in phase with the light passing through the region free from phase shifter 3 and sub-shifter 4 by etching. The light passing through only the sub-shifter 4 without passing the phase shifter 3 is delayed in phase by 180 degrees as compared with the above light.
The method of repair using the FIB is described below by reference to FIG. 13. A light-shield film pattern 2 is formed on a transparent substrate 1. A transparent film pattern is formed thereon as a phase shifter 3. The phase shifter 3 has a dent defect 5 (FIG. 13(a)). The region including this defect 5 is selectively removed by ion milling using gallium (Ga) ion beam 7. At this time, the transparent substrate 1 is also etched by ion milling. The etching depth at this time is set at such a depth that the phase of the light passing through the etching region may be advanced by 180 degrees.
In the prior art shown in FIG. 12, in order to remove the defect 5 selectively, a resist is applied on the defect 5, and the specified portion is exposed and developed to form a resist pattern. Afterwards, using this resist pattern as the mask, the phase shifter 3 and the sub-shifter layer 4 are dry etched, and then the resist is removed. In this method, however, the mask repair process is complicated.
The phase shift mask repair method shown in FIG. 13 involves problems in the etching depth or flatness by etching of the ion beam 7, and controllability of etching shape, among others. Herein, the etching depth must be within a fluctuation of 10%, in phase difference, and the depth fluctuation must be (within 5%. The light transmissivity in the etching region is required to be 95%, or more. That is, after etching, the substrate surface must be flat and transparent. Besides, the film thickness to be etched determines the phase difference applied to the transmission light, and hence it is necessary to etch accurately and uniformly. At the present, it is difficult to obtain such precision by the processing with ion beam 7.